1. Field of the Invention
The invention relates to a metering pump having a synchronous motor that is stepped down via a mechanical gear and the rotating movement of the motor is converted into a stroke movement that drives a membrane pump.
2. Description of the Related Art
With metering pumps, one differentiates essentially between two types of construction, electromagnetically and electromotorically driven ones. With the first construction type, the control of the delivery quantity is effected via the frequency at which the stroke magnet is driven, wherein additionally the stroke volume as a rule may be changed by way of a mechanical setting screw, i.e., the delivery quantity per operating cycle. In contrast, with electromotorically driven metering pumps, such a mechanical stroke adjustment is not regularly provided and it would indeed be technically comparatively complicated. With these pumps, the oscillating movement of the pump is produced by a gear in the manner of a crank mechanism or a suitable block guiding, wherein the rotational movement of the motor is usually stepped down in order to permit a more fine touch control of the pump. With the application of stepper motors the interval between the motor steps for this is accordingly varied. The latter construction type is however relatively expensive on account of the motor and furthermore demands a complicated electronic control.
It is usual to provide an impulse control in order to adapt the flow to be delivered by the metering pump to another delivery flow (main delivery flow) with regard to time and quantity, in order to ensure a predetermined mixing ratio between these flows. For this metering pumps of both construction types usually have an input for an external impulse generator. At the same time a metering quantity to be set previously, i.e. a certain volume to be delivered by the metering pump, is allocated to each impulse of the external impulse generator which emits impulses in temporal intervals according to the throughput quantity of the main delivery flow. Whilst with the initially mentioned metering pumps with an electromagnetic drive this metering quantity to be delivered per impulse is usually set by the mechanical stroke adjustment, with electromotorically driven metering pumps this is exclusively possible by changing the stroke speed, thus the rotational speed of the drive motor.
Stepper motors and servo-motors with the controls necessary for this, as well as electric motors with a controllable rotational speed are comparatively expensive. One therefore makes do without the possibity of external impulse control with metering pumps with an electromotoric drive without rotary speed variation, thus those metering pumps of a simple construction type.